Location Based Handoff for Mobile Devices

ABSTRACT

Mobile devices ( 12   1   , 12   2   , . . . , 12   n ) are handed off in overlapping cells ( 18   1   , 18   2   , . . . , 18   n ) by tracking a velocity (speed and direction) of movement of any number of the mobile devices ( 12   1   , 12   2   , . . . , 12   n ). Access points ( 14   1   , 14   2   , . . . , 14   n ) are each located in an associated operational cell ( 18   1   , 18   2   , . . . , 18   n ) and each operates at a dedicated frequency. The initial signal strengths are measured in advance at predefined locations within a defined space ( 16 ) to define signal strengths as location map ( 42 ). A nearby initial access point (typically the nearest) and at least two nearby (typically next nearest) access points are assigned for one of the mobile devices ( 12   1   , 12   2   , . . . , 12   n ), and the frequencies of each access point identified. A tracking processor ( 50 ) periodically scans the frequencies of the nearby access points to determine the location velocity of the mobile device and predict when a handoff will be appropriate. E.g., the actual signal strength measurements between the mobile device and the nearby access points are collected and analyzed to determine a proper time for a handoff of one or more of the nearby access points.

The present invention relates to the communications arts. It finds particular application in the wireless local area network systems (WLAN) and will be described with particular reference thereto. However, the invention may also find application in other communications systems.

WLAN is a flexible data communications system implemented as an extension to, or as an alternative for, a wired local area network (LAN). Typically, WLAN uses radio frequency (RF) technology to transmit and receive data over the air without relying on any physical connection. The data being transmitted is superimposed on the radio carrier so that it can be accurately extracted at the receiving end. Multiple radio carriers exist in the same space at the same time without interfering with each other, provided that the radio waves are transmitted on different radio frequencies. To extract data, a radio receiver tunes in one frequency while rejecting all other frequencies.

In a typical WLAN system, a transmitter/receiver device, called an access point, connects to the wired network from a fixed location using standard cabling. Typically, the access point receives, buffers, and transmits data between the WLAN and the wired network infrastructure. Generally, a single access point supports a small group of users and can function in a range of approximately thirty to fifty meter radius. End users access the WLAN through WLAN adapters, which are implemented as PC cards in notebook or palmtop computer, as cards in desktop computers, or integrated within hand-held computers. WLAN adapters provide an interface between the client network operating system (NOS) and the airways via antenna.

Generally, in a large facility such as an office, a hospital, a manufacturing facility, or the like, it is necessary to install more than one access point. The access points are installed to blanket the area with overlapping coverage cells so that the clients can roam seamlessly throughout the area without ever loosing contact. When a client moves from one area to another, a handoff has to be performed to assign new system resources associated with the new access point. The handoff involves executing a set of negotiations between the mobile device and a central computer. As a result, the client is handed over from one access point to another in a way that is invisible to the client. Generally, a handoff improves performance of the system at the expense of tying up more system resource.

Efficient and timely handoff procedures are very important within the WLAN which has small operating cells and great demand for increased communications system capacity. A typical approach for a mobile device handoff is to scan all radio frequencies within the WLAN operational space to determine a relative strength of the mobile device signal. However, with many users being hooked up to the WLAN and numerous crossings of cells boundaries, this approach considerably ties up system resources and slows down communications throughput.

There is a need for technique that provides adaptive, fast, efficient, seamless, and cost effective handoff of mobile units in WLAN. The present invention contemplates a new and improved method and apparatus that overcomes the above-reverenced problems and others.

In accordance with one aspect of the present invention, a communications system is disclosed. A plurality of mobile wireless units and a plurality of access points, each surrounded by an associated operational cell, are located within a defined area of a WLAN. The operational cells are overlappingly disposed in the defined space. Each access point operates at a dedicated frequency. A means tracks a movement of at least one mobile device within the defined space. A means scans identified scanning frequencies of nearby access points to measure actual signal strengths between the at least one mobile device and each of the nearby access points. A means calculates at least a position of the at least one mobile device by comparing the actual signal strengths with a map of relative signal strengths at predefined locations in the defined space. A means assigns an access point with a strongest signal to the at least one mobile device based on its location and the map of relative strengths in the defined space.

In accordance with another aspect of the present invention, a method for handing off at least one mobile device from one access point to another in a wireless local area network is disclosed. A movement of the at least one mobile device within the defined space is tracked. Identified scanning frequencies corresponding to each of an identified plurality of nearby access points are scanned. Actual signal strengths at each of the identified frequencies between the at least one mobile device and the identified access points are measured. At least a position of the at least one mobile device is calculated by comparing the actual signal strengths with a map of relative signal strengths at predefined locations in the defined space. An access point with a strongest signal is assigned to the at least one mobile device based on the calculated position and the map.

One advantage of the present invention resides in the fast and efficient handoff of the wireless mobile units in the wireless local area network.

Another advantage resides in the cost effective handoff.

Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.

The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention.

FIG. 1 schematically shows a portion of a WLAN communications system; and

FIG. 2 schematically shows a part of a WLAN communications system in accordance with the present application.

With reference to FIG. 1, a wireless local area network 10 includes one or more mobile devices or units 12 ₁, 12 ₂, . . . , 12 _(n). Preferably, the mobile devices 12 ₁, 12 ₂, . . . , 12 _(n) are palm computers, notebook computers, held-hand devices, PDAs, pagers, desktop computers, or any other devices which can be configured for wireless communications. Generally, the network 10 couples multiple access points or stations 14 ₁, 14 ₂, . . . , 14 _(n) (only six access points are shown for simplicity of illustration), which are distributed throughout a defined area or space 16 to provide wireless service to the mobile devices 12 ₁, 12 ₂, . . . , 12 _(n) which operate within the space 16 and are configured to communicate with the access points 14 ₁, 14 ₂, . . . , 14 _(n). Each access point 14 ₁, 14 ₂, . . . , 14 _(n) has a finite operational range or access point cell 18 ₁, 18 ₂, . . . , 18 _(n), which is typically 30-50 meters. Each access point 14 ₁, 14 ₂, . . . , 14 _(n) operates within its own dedicated radio channel with a known radiofrequency. Because the operational ranges 18 ₁, 18 ₂, . . . , 18 _(n) overlap within the area 16, each channel operates at a unique radiofrequency to prevent concurrent communications of the mobile device 12 ₁, 12 ₂, . . . , 12 _(n) with more than one access point at the same frequency. Of course, it is also contemplated that other WLAN designs can be used, in which the frequencies can be reused.

The access points 14 ₁, 14 ₂, . . . , 14 _(n) are wired or otherwise connected into a wired network infrastructure or a local area network 20. A central computer 22, which is connected to the local area network 20 and includes associated software means 24 and hardware means or processor 26, oversees the operations of the WLAN system 10 and, preferably, provides an interface to various systems and/or applications which are available within the local area network 20.

Each access point 14 ₁, 14 ₂, . . . , 14 _(n) includes an antenna or receiving/transmitting means 30 to communicate bi-directionally with the mobile devices 12 ₁, 12 ₂, . . . , 12 _(n). E.g., the access points 14 ₁, 14 ₂, . . . , 14 _(n) at least receive, buffer, and transmit data between the mobile devices 12 ₁, 12 ₂, . . . , 12D and the wired network 20. Each mobile device 12 ₁, 12 ₂, . . . , 12 _(n) includes associated hardware means 32 and software means 34. The hardware and software means 32, 34 are implemented or integrated into the mobile devices 12 ₁, 12 ₂, . . . , 12 _(n) to provide an interface between the mobile devices 12 ₁, 12 ₂, . . . , 12 _(n) and the receiving/transmitting means 30.

With continuing reference to FIG. 1, when the mobile device 12 ₁ is in the cell 18 ₁, it communicates with the access point 14 ₁. As the mobile device 12 ₁ moves within the defined area 16, the processor 26 executes a set of instructions and, if the handoff is determined to be necessary, handoffs the mobile unit 12 ₁ to another access point as will be discussed in a greater detail below.

With reference to FIG. 2, an initial signal strengths determining means or computer routine or algorithm 38 determines initial signal strengths, i.e. signal gradients, at predefined locations. Optionally, the initial signal strengths determining means 38 defines the initial signal strengths at the predefined locations statistically. A mapping means 40 maps the initial signal strengths into the defined space 16. A two- or three-dimensional geographical signal strength map of the area 16 indicating all of the stations or access points 14 ₁, 14 ₂, . . . , 14 _(n), associated operational frequencies, and the defined initial signal strengths are stored in a database or an area map memory 42. Preferably, the database 42 is located at the central computer 22. Optionally, the database 42 is located within the WLAN 10, e.g. at the mobile device 12 ₁, 12 ₂, . . . , 12 _(n).

A handoff means or computer routine or algorithm 48 executes an access point assignment and subsequent handoff to the closest access points for determined locations and identifies a channel with the strongest signal, e.g. the best signal to noise ratio, of the mobile devices 12 ₁, 12 ₂, . . . , 12 _(n) within the defined space 16. More specifically, when one of the mobile units 12 ₁, 12 ₂, . . . , 12 _(n), such as the mobile unit 12 ₁, is powered up, a position determining means or computer routine or algorithm 50 determines the location of the mobile device 12 ₁ within the defined space 16. A scanning means or computer routine or algorithm 52 scans operational frequencies of the channels of all the access points within the defined space 16 to determine gradients of actual signal strengths between the mobile device 12 ₁ and each of the scanned frequencies. The scanned frequencies are presented in an order of the gradients, e.g. from the highest gradient to lowest, and are stored in a scanned frequencies memory 54. A position calculating means or computer routine or algorithm 56 calculates a position of the mobile device 12 ₁, e.g. the initial position P1, by comparing the actual signal strengths against the mapped signal strengths. Preferably, the position is determined from the relative signal strengths of the three strongest frequencies, normally the three closest stations. However, other numbers of frequencies/stations for determining the position of the mobile units can be used. A larger number gives greater positional accuracy. In some instances, less than three may identify the location of the mobile unit uniquely, particularly when walls and other physical obstructions are considered.

The determined position P1 is stored in a position memory 58. An access point assigning means or computer routine or algorithm 60 identifies a location of an access point with the strongest signal and assigns it to the mobile device 12 ₁. A scanning frequencies identifying means or computer routine or algorithm 62 identifies frequencies of, preferably three adjacent channels, e.g. three access points 14 ₁, 14 ₃, 14 ₄ which are closest to the position P1 which typically have the strongest signals. The three adjacent access points and corresponding frequencies of each associated channel are stored in a scanning frequencies memory 64.

As the mobile device 12 ₁ moves from the position P1 towards position P2, the signal strength between the mobile device 12 ₁ and the access point 14 ₁ is declining and signal strength between the mobile device 12 ₁ and the access points 14 ₃ and 14 ₄ is increasing. With the defined space 16 being mapped, the scanning means 52 periodically scans the three stored adjacent frequencies to collect the signal strength values between the mobile device 12 ₁ and the frequencies of the three closest access points as the mobile device 12 ₁ moves from the position P1 toward the position P2. The position calculating means 56 calculates a new position of the mobile device 12 ₁. Preferably, the exact position of the mobile device 12 ₁ is triangulated by comparing the actual signal strengths at the three measured frequencies. A velocity determining means or computer routine or algorithm 66 compares location results of the periodic scanning and determines speed and direction of movement of the mobile device 12 ₁ or any other mobile device within the defined space 16. Based on the speed and direction of the mobile device, a future position predicting means or computer routine or algorithm 68 predicts future positions of the mobile device as well as projected future signal strengths between the mobile device and access points of the cell in which the mobile device is located and adjacent cells.

If it is determined that the mobile device 12 ₁ is approaching a new position, e.g. the position P2, at which the map 42 shows a different access point will have stronger signal, the access point assigning means 60 assigns the mobile device 12 ₁ a new primary communication access point which, preferably, has the strongest signal. By consulting the map 42, the scanning frequencies identifying means 62 identifies the frequencies of the, preferably, three access points 14 ₁, 14 ₄, 14 ₆ that provide the strongest signals with the new position P2. In this example, the three access points which measure location stay the same, but the access point through which communication occurs shifts from 14 ₁ to 14 ₃.

As the mobile device moves toward location P3, one of the three location measurement access points changes, i.e., the three closest change from 14 ₁, 14 ₃, and 14 ₄ to 14 ₃, 14 ₄, and 14 ₆. The change over from access point 14 ₁ to 14 ₆ is preferably done at the time which the position predicting means predicts the mobile device will become closer to access point 14 ₆ than to access point 14 ₁. In this manner, if the mobile device is moving so fast compared to the location sampling interval that it will be out of range of access point 14 ₁, it will still seamlessly triangulate its position off of access points 14 ₃, 14 ₄, and 14 ₆. The three new adjacent access points and corresponding frequencies of each associated channel are stored in the scanning frequencies memory 64 to be periodically analyzed by the position determining means 50 to monitor the position of the mobile device 12 ₁. The AP assigning means 60 accesses the map 42 and the scanning frequencies memory 64 to retrieve the frequencies of the new access point 14 ₆ and the discontinued access point 14 ₁. This frequency change is communicated to the mobile device to cause a seamless change in the scanning frequencies without searching all frequencies looking for the new closest access points.

With continued movement to P3, the communication frequency is changed from the frequency of access point 14 ₃ to the frequency of access point 14 ₆.

In some circumstances, the access point assigning means 62 may not assign a mobile unit to the access point with the strongest signal. The access point assigning means 60 confers with an arbitration means or computer algorithm 70 to determine the best overall distribution of the assignment of the mobile devices to the access point. In one example, the arbitration means 70 looks to see if any access point is over-crowded or approaching capacity. If a mobile unit is approaching a near capacity access point, the handoff is deferred until the map 42 shows it is moving beyond a signal strength of its current access point or another mobile device moves out of the over-crowded access point's region. As a second example, the arbitration means 70 projects how long the mobile unit will be in the new access point region from the trajectory of its projected velocity and the map. If the projected trajectory will only pass briefly through the new zone without loosing satisfactory signal strength from its current access point or the next yet projected access point, the handoff from the current access point to the next access point and from the next access point to the next yet access point can be skipped in favor of a handoff directly from the current access point to the third next yet access point.

With continuing reference to FIG. 2 and further reference to FIG. 3, to improve the accuracy and reliability of position tracking, more than three scanning frequencies of the neighboring access points are selected for scanning to track the position of the mobile device 12 ₁, 12 ₂, . . . , 12 _(n). The number of the adjacent access points (scanned frequencies) varies as a function of the certainty of the position accuracy to minimize the number of channels to be scanned to determine the accurate position of the mobile unit 12 ₁ as it moves within the defined area 16. As position becomes less certain, more nearby access points are scanned; as the position becomes more certain, fewer access points are scanned.

More specifically, as the position calculating means 56 determines the position of the mobile device 12 ₁, a certainty determining means 72 determines a certainty of accuracy of the determined location based on prespecified factors such as longer intervals between samplings, motion at high rate of speed, motion along an erratic trajectory, or the like. The determined certainty is compared with a threshold which is preferably predetermined in advance based on certain criteria. Of course, it is also contemplated that the threshold can be varied to restrict or increase the number of scanned frequencies based on the system requirements. If the determined certainty is below the threshold, the scanning frequencies identifying means 62 accesses the area map database 42 and selects all operational frequencies existing within the area 16 for scanning. The scanned frequencies are presented in an order of the gradients, e.g. from the highest gradient to the lowest, and are stored in the scanned frequencies memory 54. The position calculating means 56 recalculates the position of the mobile device 12 ₁, and the certainty determining means 72 determines the certainty of the recalculated position. The redetermined certainty is compared with the threshold. If the determined certainty is still below the threshold, the position calculating means 56 requests more frequencies from the scanned frequencies memory 54. The certainty determining means 72 recalculates the certainty each time an additional frequency is added to the position calculation and compares the determined certainty with the threshold until the threshold is reached and/or exceeded. The identified optimal adjacent access points and corresponding frequencies are stored in the scanning frequencies memory 64 to be scanned by the scanning means 52 to determine the speed and position of the mobile device 12 ₁ with more accuracy.

The invention has been described with reference to the preferred embodiments. Modifications and alterations will occur to others upon reading and understanding of the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof. 

1. A communications system comprising: a plurality of mobile wireless units located within a defined space of a wireless local area network; a plurality of access points disposed at known locations in the defined space, each access point operating at a dedicated frequency; a means for tracking movement of at least one mobile device within the defined space including: a means for scanning identified scanning frequencies of nearby access points to measure actual signal strengths between the at least one mobile device and each of the nearby access points, and a means for calculating at least a location of the at least one mobile device by comparing the actual signal strengths with a map of relative signal strengths at predefined locations in the defined space, and a means for assigning the nearby access points with strongest signals to the at least one mobile device based on its location and the map of relative strengths in the defined space.
 2. The system as set forth in claim 1, wherein the scanning means only scans the frequencies of the assigned nearby access points.
 3. The system as set forth in claim 1, wherein the tracking means tracks the movement of the at least one mobile device by periodically scanning the frequencies of three assigned access points adjacent the calculated location.
 4. The system as set forth in claim 1, wherein the position tracking means includes: a velocity estimating means for determining speed and direction of movement of the at least one mobile device.
 5. The system as set forth in claim 3, wherein the tracking means includes: a means for predicting future locations of the mobile device, the assigning means assigning the nearby access points based on the predicted location.
 6. The system as set forth in claim 1, further including: a means for determining a degree of certainty of an accuracy of the calculated location.
 7. The system as set forth in claim 6, wherein the number of nearby access points is a function of location accuracy certainty and the tracking means tracks the movement of the at least one mobile device by periodically scanning a variable number of frequencies.
 8. The system as set forth in claim 1, further including: a means for measuring a plurality of initial signal strengths at predefined locations within the defined space; a means for mapping the initial signal strengths in relation to predefined locations in the defined space; and a means for identifying locations and scanning frequencies of the access points in the defined space.
 9. In a wireless local area network, a method for handing off at least one mobile device from one access point to another, the method comprising: tracking a movement of the at least one mobile device within the defined space including: scanning identified scanning frequencies corresponding to each of an identified plurality of nearby access points, measuring actual signal strengths at each of the identified frequencies between the at least one mobile device and the identified access points, and calculating at least a location of the at least one mobile device by comparing the actual signal strengths with a map of relative signal strengths at predefined locations in the defined space; and assigning nearby access points with strongest signals to the at least one mobile device based on the calculated location and the map.
 10. The method as set forth in claim 9, further including: identifying frequencies dedicated to access points which are nearest to the mobile device; and tracking the movement of the at least one mobile device by periodically scanning only the frequencies of the currently identified nearest access points.
 11. The method as set forth in claim 10, further including: tracking the movement of the at least one mobile device by periodically scanning the frequencies of the three nearest access points.
 12. The method as set forth in claim 10, further including: updating the frequencies of the nearest access points as the mobile device changes location.
 13. The method as set forth in claim 9, further including: estimating at least a speed and a direction of movement of the mobile device; predicting the mobile device location from the estimated speed and direction; and reassigning the nearest access points based on the predicted location and the map.
 14. The method as set forth in claim 9, further including: measuring a plurality of initial signal strengths at predefined locations within a defined space; mapping the initial signal strengths in relation to locations in the defined space; and identifying a plurality of locations and scanning frequencies of the access points located in the defined space.
 15. The method as set forth in claim 14, further including: determining a certainty of an accuracy of the calculated location of the mobile device.
 16. The method as set forth in claim 15, further including: based on the certainty of the location calculation accuracy, adjusting a number of nearest access points whose frequencies are scanned; and tracking the movement of the mobile device by periodically scanning the frequencies of the currently nearest access points.
 17. The method as set forth in claim 15, further including: comparing the determined certainty with a requested threshold.
 18. The method as set forth in claim 17, further including: (a) in response to the certainty being below the requested threshold, scanning the scanning frequencies of a large number of the access points located in the defined space; (b) measuring actual signal strengths at each of the scanning frequencies between the at least one mobile device and the corresponding access point; (c) organizing the measured signal strengths in a categorized list; (d) recalculating the location of the at least one mobile device; (e) recalculating a certainty of an accuracy of the recalculated location of the mobile device; and (f) comparing the recalculated certainty with the requested threshold.
 19. The method as set forth in claim 18, further including: in response to the recalculated certainty being greater than the requested threshold, selecting at least three access points from the categorized list based on signal strengths.
 20. The method as set forth in claim 18, further including: in response to the recalculated certainty being below the requested threshold, measuring the number of the scanning frequencies from the categorized list; repeating steps (d)-(f) until the threshold is exceeded; and identifying a set of optimal scanning frequencies.
 21. The method as set forth in claim 15, wherein a number of nearest access points is a variable based on the determined certainty of the location calculation accuracy.
 22. The method as set forth in claim 9, wherein the frequencies of the nearby access points are different.
 23. The method as set forth in claim 9, further including: handing off a plurality of mobile devices in the defined space; evaluating an overall distribution of the mobile devices in the defined space to determine a capacity of each access point; and assigning the nearest access points to each mobile device based at least on both the determined capacity and the actual signal strength.
 24. A communications system comprising: a plurality of mobile wireless units located within a defined space of a wireless local area network; a plurality of access points disposed at known locations in the defined space, each access point operating at a dedicated frequency; a computer processor for tracking movement of the mobile devices and reassigning frequencies of closest access points to each mobile device, the computer processor being programmed to perform the steps of: scanning identified scanning frequencies corresponding to each of an identified plurality of nearby access points, measuring actual signal strengths at each of the identified frequencies between the at least one mobile device and the identified access points, calculating at least a location of the at least one mobile device by comparing the actual signal strengths with a map of relative signal strengths at predefined locations in the defined space; and assigning nearby access points with strongest signals to the at least one mobile device based on the calculated location and the map. 